In conventional apparatus for drilling holes in a workpiece, the workpiece is clamped to a workpiece support on the base of the drill apparatus and the drill head is mounted for movement along its base toward and away from the workpiece support to drill the holes in the workpiece. While such apparatus worked well with workpieces that are small enough to be easily moved to proper position and clamped on the workpiece support, they are not entirely satisfactory for use with very large workpieces such as structural beams and the like. Such large workpieces cannot be readily moved to accurately position the same relative to the drill axis. Moreover, in such apparatus the stresses incurred during a drilling operation are transmitted through a stress path that includes the workpiece clamp, the workpiece support, the drill base and the drill head. It is therefore necessary to provide relatively heavy clamps for clamping the workpiece to the workpiece support as well as a heavy base and drill head supporting structure.